1. Field of the Invention
The present invention relates to systems and methods for satellite navigation, and particular to a system and method for reducing error from beacon measurements used or satellite navigation.
2. Description of the Related Art
Spacecraft typically have one or more payloads that are directed to transmit or receive energy from ground stations. For example, communication satellites include one or more uplink antennae for receiving information from an uplink center, and one or more downlink antennae for transmitting information to a terrestrial receiver. The uplink and downlink antennae are typically disposed on the satellite body (or spacecraft bus) and are directed toward a terrestrial location where an uplink/downlink antenna is transmitting/receiving the information.
In many cases, the information is beamed to a plurality of terrestrial receivers spanning a wide geographical area. In such situations, the pointing accuracy of the uplink/downlink antennae is not particularly critical. However, in other cases, spacecraft payloads must be pointed at the desired target with a high degree of accuracy. This can be the case, for example, in cases where the uplink/downlink antenna is a narrow beamwidth antenna, or when spatial diversity is critical. In such situations, spacecraft""s on-board navigation system (which relies on inertial sensors and perhaps sun, earth, and magnetic sensors as well) often cannot support the precise pointing requirement.
In such cases, beacon sensor systems can be used to increase payload pointing performance and spacecraft body control. The beacon sensor monitors an uplink carrier (which can also be used to provide commands to the satellite) to sense mispointing of the antenna structure. Using the beacon sensor data as a reference, the satellite navigational system parameters can be updated to improve accuracy. In particular, the beacon sensor data can be used to replace earth sensor data.
Recent technology advancement leads to the realization of digital beacons. In a digital beacon, the beacon beams are formed digitally using an on-board Digital Signal Processor (DSP). The beams are formed by selecting desired beam weights for each feed chain. However, the accuracy of the digital beacons are negatively affected by the performance limitations of the digital beam-forming technique and its implementation. Although some digital beacon sensor errors can be ameliorated by calibration and the adjustment of weighting to beacon sensor channels, asymmetry errors due to beam-forming approximation by finite number of feed chains, and quantization errors due to the finite-bit representation of the weighting factors themselves can severely impact beacon accuracy and therefore payload pointing accuracy. What is needed is a system and method for compensating for such asymmetry error and quantization errors. The present invention satisfies this need.
To address the requirements described above, the present invention discloses a method, apparatus for reducing errors in a plurality of beacon beams, and aggregate errors in a digital beacon. The method comprises the steps of computing quantized channel weights {tilde over (W)}c from channel weights Wc for at least some of the channels; estimating the quantization error xcex94Ba for each of the beacon beams from a difference between the channel weights Wc and the computed quantized channel weights {tilde over (W)}c; and adding the estimated quantization error xcex94Ba to the beacon beams. Similarly, beacon biases equivalent to the beacon asymmetry error can be computed by the ground beacon beam forming software and uploaded to the on-board software for error compensation. In one embodiment, the apparatus comprises one or more means, such as a processor, communicatively coupled to a memory storing instructions for performing these operations.